Linear regulated power supplies typically include one of many commercially available integrated circuits as the heart of a regulator control loop. The integrated circuit generally includes a built-in overcurrent protection/limiting capability to protect the power supply from damage in the case of overcurrent situations. One such commonly used integrated circuit is the industry standard 723C chip manufactured by, among others, National Semiconductor. Overcurrent protection operation of the 723C will be explained briefly in conjunction with the circuit of FIG. 1.
In FIG. 1, a load 10 is supplied with electrical power from an unregulated (AC driven) DC power supply 12. Interposed between load 10 and power supply 12 is a regulator circuit 14 that includes (for example) a 723C chip 16. Only connections to pin numbers 2, 3 and 10 of chip 16 pertain to overcurrent protection. Accordingly, only these pins are shown and discussed hereinbelow. A low value, current sensing resistor 18 is placed in series with load 10. Voltage developed across sensing resistor 18 is applied to pin numbers 2 and 3 of chip 16. If this voltage exceeds an internally set threshold value, the 723C chip 16 reacts by shutting down the base drive (from pin 10 of the 723C chip) to a pass transistor 20 thereby lowering the output voltage applied to the positive terminal of load 10 to a safe level.
If load 10 is primarily resistive, current is proportional to voltage and there is no problem with a "turn-on" transient. However, active devices such as operational amplifiers can exhibit highly nonlinear supply current characteristics at a very low (under 2 volts) supply voltages. Naturally, low supply voltage occurs on a transient basis each time power is turned on and the power supply's output voltage rises from zero. During the initial stage of this transient, an active device can draw considerably more current than it does at the correct operating voltage. Accordingly, the 723C chip 16 shuts down the output to load 10 when an overcurrent condition is sensed through sensing resistor 18. Unfortunately, without further circuitry, the output to load 10 remains in a locked-down state which is the equivalent of outputting only a few tenths of a volt.
To alleviate this problem, a series RC network 22 is typically placed between pins 2 and 3 of the 723C chip 16. Upon initial turn-on, the voltage between pins 2 and 3 is prevented from exceeding the internally set threshold value for a short interval while capacitor 24 charges. While this works satisfactorily for the initial power-on transient, it does not prevent the locked-down state from occurring in cases of momentary AC line power dropouts. Specifically, if AC line power returns before capacitor 24 has discharged, it is possible for the resultant voltage produced by the momentary overcurrent through sensing resistor 18 to be applied unattenuated across pins 2 and 3 of the 723C chip 16 thereby triggering a shutdown.
Thus, a need exists for suppressing turn-on transients for active device loads for both the initial turn-on transient and any subsequent turn-on transients caused by momentary AC power dropouts. Accordingly, an object of the present invention is to provide a turn-on transient overcurrent response suppressor for use with existing overcurrent protection circuits to prevent false triggering of the protection circuits in the event of initial or subsequent turn-on transient overcurrent conditions. Another object of the present invention is to provide a turn-on transient overcurrent response suppressor for use with existing overcurrent protection circuits to prevent the creation of a locked-down state by the existing protection circuit.